1. Technical Field
The present invention relates to combinations or mixtures of antigens which may be used in the detection of IgM or IgG antibodies to Toxoplasma gondii, as well as one antigen, in particular, which may be used to distinguish between acute and chronic infection. Furthermore, the present invention also relates to methods of using these combinations of antigens, antibodies raised against these combinations of antigens or against the novel P29 antigen thereof, as well as kits and vaccines containing the antigens present in the combinations.
2. Background Information
Toxoplasma gondii is an obligate intracellular parasite which is classified among the Coccidia. This parasite has relatively broad host range infecting both mammals and birds. The organism is ubiquitous in nature and exists in three forms: tachyzoite, cyst, and oocyst (Remington, J. S., McLeod, R., Desmonds, G., Infectious Diseases of the Fetus and Newborn Infant (J. S. Remington and J. O. Klein, Eds.), pp. 140-267, Saunders, Philadelphia (1995)). Tachyzoites, found during acute infection, are the invasive form capable of invading all nucleated mammalian cells. After the acute stage of infection, tissue cysts called bradyzoites are formed within host cells and persist within the host organism for the life of the host. Cysts are important in transmission of infection, especially in humans, as the ingestion of raw or undercooked meat can result in the ingestion of bradyzoites which can infect the individual resulting in an acute infection. Oocysts represent a stage of sexual reproduction which occurs only in the intestinal lining of the cat family from which they are excreted in the feces.
A T. gondii infection acquired through contaminated meat or cat feces in a healthy adult is often asymptomatic. In pregnant women and immunosuppressed patients, the clinical outcome can be very serious. An acute infection with T. gondii acquired during pregnancy, especially during the first trimester, can result in intrauterine transmission to the unborn fetus resulting in severe fetal and neonatal complications, including mental retardation and fetal death. Recrudesence of a previous T. gondii infection or an acute infection in an immunosuppressed individual can be pathogenic. Toxoplasmic encephalitis is a major cause of morbidity and mortality in AIDS patients. Toxoplasma infection has also been shown to be a significant cause of chorioretinitis in children and adults.
Diagnosis of infection with T. gondii may be established by the isolation of T. gondii from blood or body fluids, demonstration of the presence of the organism in the placenta or tissues of the fetus, demonstration of the presence of antigen by detection of specific nucleic acid sequences (e.g., DNA probes), or detection of T. gondii specific immunoglobulins synthesized by the host in response to infection using serologic tests.
The detection of T. gondii specific antibodies and determination of antibody titer are important tools used in the diagnosis of toxoplasmosis. The most widely used serologic tests for the diagnosis of toxoplasmosis are the Sabin-Feldman dye test (Sabin, A. B. and Feldman, H. A. (1948) Science 108, 660-663), the indirect hemagglutination (IHA) test (Jacobs, L. and Lunde, M. (1957) J. Parasitol. 43, 308-314), the IFA test (Walton, B. C. et al. (1966) Am. J. Trop. Med. Hyg. 15, 149-152), the agglutination test (Fondation Merieux, Serologie de I'Infection Toxoplasmique en Particulier a Son Debut: Methodes et Interpretation des Resultants, Lyon, 182 pp. (1975)) and the ELISA (Naot, Y. and Remington, J. S. (1980) J. Infect. Dis. 142, 757-766). The ELISA test is one the easiest tests to perform, and many automated serologic tests for the detection of Toxoplasma specific IgM and IgG are commercially available.
The current tests for the detection of IgM and IgG antibodies in infected individuals can vary widely in their ability to detect serum antibody. Hence, there is significant inter-assay variation seen among the commercially available kits. The differences observed between the different commercial kits are caused primarily by the preparation of the antigen used for the serologic test. Most kits use either whole or sonicated tachyzoites grown in tissue culture or in mice which contain a high proportion of extra-parasitic material, for example, mammalian cells, tissue culture components, etc. Due to the lack of a purified, standardized antigen or standard method for preparing the tachyzoite antigen, it is not surprising that inter-assay variability exists resulting in different assays having different performance characteristics in terms of assay sensitivity and specificity.
Given the limitations of serologic tests employing the tachyzoite antigen, as described above, as well as the persistent problems regarding determination of onset of infection, purified recombinant antigens obtained by molecular biology are an attractive alternative in that they can be purified and standardized. In the literature, a number of Toxo genes have been cloned and expressed in a suitable host to produce immunoreactive, recombinant Toxo antigens. For example, the Toxo P22 (SAG2), P24 (GRA1), P25, P28 (GRA2), P30 (SAG1), P35 (mentioned above), P41 (GRA4), P54 (ROP2), P66 (ROP1), and the Toxo P68 antigens have been described (Prince et al. (1990) Mol. Biochem. Parasitol 43, 97-106; Cesbron-Delauw et al. (1989) Proc. Nat. Acad. Sci. 86, 7537-7541; Johnson et al. (1991) Gene 99, 127-132; Prince et al. (1989) Mol. Biochem. Parasitol. 34, 3-13; Burg et al. (1988) J. Immunol. 141, 3584-3591; Knapp et al. (1989) EPA 431541A2; Mevelec et al. (1992) Mol. Biochem. Parasitol. 56, 227-238; Saavedra et al. (1991) J. Immunol. 147, 1975-1982); EPA 751 147).
It is plausible that no single Toxo antigen can replace the tachyozoite in an initial screening immunoassay for the detection of Toxo-specific immunoglobulins. This may be due to several reasons. First, the antibodies produced during infection vary with the stage of infection, i.e., the antibodies produced by an infected individual vary over time reacting with different epitopes. Secondly, the epitopes present in a recombinant antigen may be different or less reactive than native antigen prepared from the tachyzoite depending on the host used for expression and the purification scheme employed. Thirdly, different recombinant antigens may be needed to detect the different classes of immunoglobulins produced in response to an infection, e.g., IgM, IgG, IgA and IgE.
In order to overcome the limitations of the tachyzoite antigen in terms of assay specificity and sensitivity, a search was begun for novel Toxo antigens which could be used in combination with known existing antigens in order to configure new assays for the detection of Toxo-specific immunoglobulins.
Additionally, it should be noted that the presence of IgG antibodies in a single sample of serum is sufficient to establish that the patient has been infected but does not give an indication as to when the infection occurred. In the United States, there is no systematic serological screening program in pregnant women, whereas in countries such as France and Austria, sera are obtained at regular intervals throughout gestation in women who are seronegative when first tested. In the United States, a decision regarding whether the woman was recently infected, thereby placing her fetus at risk, is often made from results in a single sample of serum. However, it is critical in pregnant women to determine as accurately as possible if they acquired their infection just prior to or during gestation. For this reason, the presence of IgG antibodies in a pregnant woman often leads to additional serological testing to attempt to determine if the infection was acquired during pregnancy or in the distant past (Remington et al., 1995, Toxoplasmosis, 4.sup.th ed., Coord. Ed., Remington, J. S., W. B. Saunders, Philadelphia, Pa.). of the recommended additional serological tests, those that demonstrate the presence of IgM antibodies are most frequently used. However, since IgM antibodies may remain detectable for more than one year after initial infection, demonstration of these antibodies cannot be used to prove recently acquired infection (Liesebfeld et al., Journal of Clinical Microbiology 35:174-78 (1997); Wilson et al., Journal of Clinical Microbiology 35:3112-15 (1997); Wong et al., Clinical Infectious Diseases 18:853-62 (1994)). Because accurate diagnosis of the recently acquired infection in pregnant women is important for clinical management of both the mother and her fetus, a search has continued for better diagnostic methods (Remington et al., 1995, Toxoplasmosis, 4.sup.th ed., Coord. Ed., J. S. Remington, W. B. Saunders, Philadelphia, Pa.; Wong et al., supra).
In previous studies, it was observed that a 35 kDa antigen was detected in immunoblots of tachyzoite extracts probed with serum from individuals early after they became infected with T. gondii and postulated that this antigen might prove useful for detection of the acute stage of the infection (Potasman et al., Journal of Infectious Diseases 154:650-57 (1986); Potasman et al., Journal of Clinical Microbiology 25:1926-31 (1987)). Thus, a gene in the GenBank sequence database for T. gondii putatively identified as "P35" was selected for cloning, expression, and evaluation of a corresponding recombinant protein for its capacity to detect serum antibodies during the early phase of the infection. This antigen will be described in further detail below.
Additionally, it was determined that a portion of one of these antigens (i.e., P35) could be utilized to distinguish between acute and chronic infection.